In a conventional fossil fuel-fired (e.g., coal-fired) power generating unit a fossil fuel/air mixture is ignited in a boiler. Large volumes of water are pumped through tubes inside the boiler, and the intense heat from the burning fuel turns the water in the boiler tubes into high-pressure steam. The high-pressure steam from the boiler passes into a turbine comprised of a plurality of turbine blades. Once the steam hits the turbine blades, it causes the turbine to spin rapidly. The spinning turbine causes a shaft to turn inside a generator, creating an electric potential. It should be understood that as used herein, the terms “power plant” and “plant” refer to a group comprised of one or more power generating units. Each power generating unit includes systems for generation of electrical power.
Boiler combustion or other characteristics of a fossil fuel-fired power generating unit, are influenced by dynamically varying plant parameters including, but not limited to, operating conditions, boiler configuration, slag/soot deposits, load profile, fuel quality and ambient conditions. Changes to the business and regulatory environments has increased the importance of dynamic factors such as fuel variations, performance criteria, emissions control, operating flexibility and market driven objectives (e.g., fuel prices, cost of emissions credits, cost of electricity, etc.).
In most cases, efficient combustion and soot cleaning are treated as separate operational issues, typically having separate control systems, that may be evaluated and adjusted on a periodic basis under controlled settings. The high interrelationship between efficient combustion and soot cleaning demands an optimal combination of dynamically varying plant parameters and cleanliness strategies to achieve the most benefits. The techniques are best to be adaptive to accommodate volatile operating conditions and fuel variations, and must be automated to provide both tangible benefits around-the-clock and to minimize burdening power plant operators.
One of the effects of burning fossil fuels in boilers is the buildup of soot and slag on the heat transfer surfaces within the boiler. Soot and slag buildup causes a redistribution/reduction of the heat transferred, and hence heat absorbed across various sections of the boiler. In addition, it affects furnace gas flows that, for example, may cause metal temperature hot spots, imbalances in reheat or superheat sections, or possible tube erosion. This condition, if left uncontrolled, often leads to a heat rate penalty, increased NOx emissions and reduced tube/metal life. Adverse heat rate impacts arise from numerous factors including, but not limited to: (1) incomplete combustion or related effect, (2) unbalanced steam generation, (3) excessive use of desuperheater sprays, (4) high exit gas temperatures, (5) forced deslagging events, (6) increased gas flow losses, as well as (7) excessive use of soot cleaning energy. It is well documented that thermal NOx generation is largely a function of temperatures within and around the combustion zone. As the boiler sections become excessively slagged, heat transfer ability is impaired which leads to higher temperatures and higher NOx levels.
As used herein, “heat rate” refers to the number of units of total thermal input (i.e., fuel heat input) required to generate a specific amount of electrical energy (i.e., electrical power output). Heat rate provides a measure of thermal efficiency and is typically expressed in units of Btu/kWh in North America.
Incorrect accumulation of soot (i.e., fouling) of the boiler leads to poor efficiencies due to the fact that heat that could normally be transferred to the working transfer medium remains in the flue gas stream and exits to the environment without beneficial use, or transfers its energy in a less desired, less efficient portion of the power generating unit. This loss in efficiency translates to higher consumption of fuel for equivalent levels of electricity generation, which in turn may produce more gaseous, particulate, and other emissions. Another less obvious problem owing to fouling is the intensity of peak temperatures within and around a combustion zone. Total NOx generation is often a function of fuel-bound, prompt and thermal NOx. The levels of oxygen predominately influence fuel-bound NOx, which may comprise 20%-40% of the total NOx generated. Thermal NOx, which may comprise approximately 20%-50% of the total NOx, is a function of temperature. As the fouling of the boiler increases and the rate of heat transfer decreases, peak temperature increases, and so does the thermal NOx production.
Due to the composition of the fossil fuel, particulate matter (PM) is also a by-product of fuel combustion. Modern day boilers may be fitted with electrostatic precipitators (ESP) to aid in the collection of particulate matter. Although extremely efficient, these devices are sensitive to rapid changes in inlet mass concentration as well as total mass loading. Without extreme care and due diligence, excursions or excessive soot can overload an ESP, resulting in the release of high levels of PM. Furthermore, the ESP efficiency can be adversely affected by an increase in gas temperature at the point of particulate collection.
Fossil fuel-fired power generating units employ soot cleaning devices including, but not limited to, sootblowers, sonic devices, water lances, and water cannons or hydro-jets. These soot cleaning devices use steam, water or air to dislodge slag and clean surfaces within a boiler. The number of soot cleaning devices on a given power generating unit can range from several to over a hundred. Random manual, manual sequential, and time-based sequencing of soot cleaning devices have been the traditional methods employed to improve cleanliness within boilers. These soot cleaning devices are generally automated and are initiated by a master control device. In most cases, the soot cleaning devices are activated based on predetermined criteria, established protocols, sequential methods, time-based approaches, operator judgment, or combinations thereof. These methods result in indiscriminate cleaning of the entire boiler or sections thereof, regardless of whether sections are already clean. The time-based methods are now slowly being replaced by criteria-based methods, such as cleaning the boiler in accordance with maintaining certain cleanliness levels.
Traditional methods of soot cleaning, even when they are effective in assuring that a boiler is clean, fail to optimize the heat transfer rates therein, so as to maximize its operation relative to emissions and power generating unit performance. Traditional methods may also lead to accelerated tube failure through erosion, (blowing on clean surfaces), or metal fatigue (creating large temperature differentials when a large layer of soot has built up and is cleaned).
It may appear that the goals or objectives of soot cleaning are simple and easy to ascertain, i.e., maintain the boiler heat transfer surfaces in a clean state so as to maximize heat transfer from the combustion gas side to the steam/water side of the boiler. However, the effects of boiler soot cleaning are complex and interrelated with combustion and/or power generating unit characteristics. There are multiple trade-offs between achieving a cleaner surface and the cost of operating a soot cleaning device. Furthermore, cleaning the waterwalls in the lower furnace has the desirable effect of improving heat transfer in the steam generation circuits leading to increased steam production. It also results in a reduced Furnace Exit Gas Temperature (FEGT) and reduced heat transfer in the convection pass and possible reductions in spray flows. Thus, main steam and reheat steam temperatures are reduced with an associated cost penalty manifested as an increase in unit heat rate. Trade-offs occur as furnace walls can become too clean and result in non-optimal steam temperature as a result of excessive heating in the furnace area and lead to either an effect of excessive steam attemperation spray, or conversely insufficient temperatures in one or another steam paths. Both affects can result from over cleaning or under cleaning of a furnace area.
Additionally, trade-offs must be considered when cleaning the convective pass of the boiler. Under some operating conditions, convective pass cleaning may necessitate excessive desuperheating spray in order to maintain superheat or reheat steam temperatures within operational limits. This directly results in an increase in unit heat rate. Hence, soot cleaning should be considered within the wider context of overall optimization of power generating units including the distribution of unit heat transfer as well as combustion or other system optimization including affects such as heat rate, NOx, CO, PM, metal fatigue, erosion, keeping the power generating unit in control range (e.g. spray flows or burner tilts), etc.
Operators of power plants are challenged with a number of non-linear and conflicting objectives, some of which are discussed above, while ensuring that power generating units are stable and capable of meeting system dispatch requirements. Simultaneously optimizing the objectives of NOx, PM, heat rate, opacity, or other requirements is difficult and unrealistic for a control room operator to do manually, even more so when that operator is also required to maintain control of the balance of the power generating unit's equipment. Owing to business demands and regulatory issues, present day boilers are subject to volatile changes in operation and fuel types or blends.
In recent years, the industry has been introduced to a number of “Intelligent” Rule-Based systems that derive their knowledge base from operator experiences, static design data, general thermal principles and periodic testing. Rule-based systems are only as good as the rules that drive them and established rules alone cannot accommodate the diverse set of operating conditions that may be encountered on a daily basis. Furthermore, time or rule-based systems alone are not the best answer due to the complexity of the individual components, combinations thereof and the desire to satisfy multiple objectives in a dynamic real-time environment. While the rule-based systems are an improvement over conventional hard-coded techniques, these techniques do not deal with some of the dynamic and transitional operational characteristics of present day boilers.
Examples of some current “Intelligent” Rule-Based systems for soot cleaning will now be briefly described. Advise IT Sootblowing Advisor module from ABB Inc. calculates sectional surface cleanliness values in real time, as well as the temperature of gas entering each section. The thermal principles based model is configured/tuned to unit-specific boiler data. The results obtained from this module support power plant operators and engineers in optimizing current power plant sootblowing strategy. EtaPRO software package from General Physics provides calculated cleanliness data (i.e., cleanliness parameters) for different zones in a boiler. The use of such cleanliness data may be site and application specific. A water cannon system from Clyde Bergemann provides calculated heat flux value for each zone mapped by the system. The use of such calculated heat flux data may be site and application specific.
The present invention provides a method and apparatus for optimizing the operation of a power generating plant using artificial intelligence techniques that overcome these and other deficiencies of the prior art.